>> ... Is the registration fee for the Symposium refundable ...
I suggest that you register and pay - to make sure there is room for you.
Then, if you cannot show up, contact Martha ASAP, and advise her that your registration is available to someone else. She will find a suitable recipient for your graciousness.
YOU get a tax receipt ... and someone else gets the experience. EVERYONE wins.
Clint, K6LCS